Many lasers use, in known manner, a gaseous active medium at pressures of between 0.1 and 10 atmospheres and capable of emitting over an entire range of wavelengths lying between an ultraviolet 200 nm and an infrared 10,000 nm when excited by a transverse electric discharge.
One such laser includes two linear electrodes: an anode and a cathode; which extend parallel to each other and to the emission axis of the laser. A transverse discharge, perpendicular to the emission axis of the laser, is generally triggered by a high-speed switch, eg. a spark gap or a hydrogen thyratron, enabling energy stored in capacitive components to flow into the discharge. Such capacitive components are referred to hereinafter by the phrase laser capacitor. They are high-speed, ie. they are constructed, as are their connections, in such a manner as to be able to discharge rapidly. More particularly, in a first known, high energy (more than 0.1 J of light), transverse discharge laser, the discharge is obtained by a sudden application of the laser capacitor voltage via a power switch such as a spark gap.
With this first known laser many problems are encountered when it is desired to obtain reliable industrial type operation. The high speed switch suffers from three major drawbacks. Firstly it is unreliable due to rapid wear of its switching electrodes while passing very high peak currents (about 10.sup.4 to 10.sup.6 amps). Secondly, such a switch itself consumes a large fraction (30% to 50%) of the stored energy, thereby reducing the energy efficiency of the laser. Finally, the switch possesses self-inductance which has the effect of mismatching the impedance of the discharge between the laser electrodes and the impedance of the assembly constituted by the laser capacitor and its connection components, thereby slowing down the discharge and further reducing laser efficiency.
For large volume lasers, a pre-ionizing device is often added to pre-ionize the active gas volume, thereby enabling a more uniform discharge to be obtained. In said first known laser, a pulse of X-rays generated inside the laser cathode is used to ionize the active medium prior to laser triggering, thereby ensuring that the laser discharge which follows is more uniform and more rapid. This first known laser is described in an article by S. C. Lin and J. I. Levatter which appeared in the journal Applied Physics Letters, Vol 34, p. 505 (1979).
Elsewhere, tests have recently been performed on a small laser to obtain satisfactory laser operation without a switch. More particularly, a gas laser is known which is excited by a transverse electric discharge which is triggered by a pulse of ultraviolet radiation passing through a semi-transparent wall of the cathode laser to ionize the active medium.
This second known laser has the following drawbacks:
in some active media, the discharge between the electrodes appears to be insufficiently uniform, although in other cases it is sufficiently uniform;
further, when the cross sectional area of the space occupied by the active medium is large, eg. 3 cm.times.3 cm, the ultraviolet radiation coming from the cathode is too attenuated when it reaches the vicinity of the anode, which makes it difficult to obtain a uniform discharge. However, it is necessary to obtain an exciting electrical discharge which is uniform in order to obtain good energy efficiency in the laser;
also, the semi-transparent cathode, which is subjected to ion bombardment during each excitation discharge deteriorates rapidly, thereby limiting the life of the laser, or the total energy which can be extracted therefrom.
Preferred embodiments of the present invention increase the energy efficiency of a gas laser excited by a transverse electric discharge which is itself triggered by photoionization, while at the same time making the excitation discharge more uniform and without using a power switch. Said preferred embodiments provide a laser with an increased operating rate and lifetime, eg. a laser which is capable of more than 10.sup.8 shots at a nominal rate of 1000 Hz and with an energy of about 1 J per shot.